The basic structure for hologram recording is as follows; a laser beam from a laser beam source is split into two beams, one of the split laser beams is modulated by an optical modulator in accordance with recording information to be signal light, the other of the split laser beams is made to be reference light, both the light beams are superimposed on a hologram recording medium, and interference fringes due to a change in the refractive index caused by the interference of both the laser beams are formed on the hologram recording medium, whereby information is recorded.
Reproduction of information from the hologram recording is performed as follows; the signal light is cut, only the reference light is applied to the hologram recording medium with the same position and angle of incidence as those when recording hologram, the interference fringes are formed on the recording medium, whereby reproduced light based on diffracted light corresponding to the original signal light can be obtained, and accordingly the reproduced light is detected by a sensor such as CCD (Charge Coupled Device) (refer to, for example, Holographic Data Storage by H. J. Coufal, D. Psaltis, G. T. Sincerbox p. 350, Springer Series Verlag, July 2000, Optical Science, and Optical Data Storage 2001, Proceedings of SPIE Vol. 4342 (2002) p. 567).
The hologram recording as described above is performed as follows; using a one-dimensional optical modulator in which a plurality of, for example, 1088 light-modulating pixels are arranged in one dimension, that is, for example, using GLV (Grating Light Valve) (refer to Grating Valve Technology: Update and Novel applications, for example), signal light is obtained based on a light and dark (white and black) image optically modulated from a laser beam according to information of “1”, “0” from each light-modulating pixel, and the signal light is applied to the hologram recording medium together with the reference light to perform hologram recording. In this case, when reproducing the record, reproduced light from the hologram recording medium is detected by a one-dimensional photodetector having a plurality of light-detecting portions where light-detecting elements of appropriate size are arranged at appropriate positions corresponding to the light-modulating pixels in an optical modulator.
However, in this case, when the position and angle of the hologram recording medium at the time of recording is shifted at the time of reproduction, the position of the reproduced image also shifts and so there may be some cases where the intended reproduction becomes difficult.
FIG. 11 schematically shows a relation of arrangement between the one-dimensional optical modulator 101 having a plurality of light-modulating pixels P1, P2, P3 . . . Pn and the one-dimensional photodetector 102, in which the number n of detecting elements D1, D2, D3, . . . Dn in the one-dimensional photodetector 102 is selected to be the same, for example, 1088 as that of the light-modulating pixels in the one-dimensional optical modulator 101.
However, according to the above structure, when signals “1”, “0” modulated by an optical modulator 1 are digital data supplied as ON/OFF of light for example, in the case where the one-dimensional optical modulator 101 performs modulation in such a manner that, for example, signal “1” is given to alternate pixels and signal “0” is given to the other alternate pixels among the light-modulating pixels P1, P2, P3 . . . Pn, if the shift occurs between the light-modulating pixels P (P1, P2, P3, . . . Pn) in the one-dimensional modulator 101 and the detecting elements D (D1, D2, D3 . . . Dn) in the one-dimensional photodetector 102 by, for example, one-half, “1” and “0” enter detecting elements half by half, as a result, no light and dark (white and black) image can be obtained but, for example, a uniformly gray one is obtained, which means that detection of information is impossible.
The occurrence of such shift is quite common. Specifically, because the hologram recording medium often takes the form of a disc or card and the recording and reproduction is repeated frequently, when the hologram recording medium is installed in the hologram recording apparatus or reproducing apparatus, a shift in position and a shift in angle are very likely to occur.
Moreover, the hologram recording medium is often made of organic materials and thus often requires ultraviolet-ray irradiation and heat treatment during recording or after recording; on this occasion, several % of shrinkage is often caused. Therefore, not only the above-described simple shift occurs in the reproduced position, but also one-to-one correspondence between the light-modulating pixels P in the one-dimensional optical modulator 101 and the detecting elements D in the one-dimensional photodetector 102 will collapse. Such phenomena also make the reproduction difficult or decrease the S/N.